In IC fabrication, devices such as transistors may be formed on a semiconductor wafer or substrate, which is typically made of silicon. MOSFET devices are widely used in numerous electronic apparatus, including automotive electronics, disk drives and power supplies. Generally, these apparatus function as switches and are used to connect a power supply to a load.
One of the applications in which MOSFET devices have been used is for radio frequency (RF) applications. Such “RF” MOSFET devices generally utilize standard lateral transistors. See, for example, the lateral MOSFET device described in U.S. Pat. No. 5,949,104, as well as the device illustrate in FIG. 1. Such lateral MOSFET devices often have a diffused source that allows a backside contact for improved thermal and parasitic reductions.
Recent advances in lateral (or laterally-diffused) MOSFET (LDMOS) devices have improved the performance and cost characteristics of lateral MOSFET devices when compared to vertical MOSFET devices for RF power amplifiers in base stations applications. Such RF LDMOS devices have been particularly useful for wireless base station applications. The RF vertical (or vertically-diffused) VDMOS structure unfortunately suffers from certain limitations relative to the LDMOS such as high output capacitance (which decreases efficiency), decreased power gain, narrowing of the usable bandwidth, and source inductance that decreases the operating efficiency.
It has been proposed to use a trench gate in place of the lateral gate so often used in RF MOSFET devices. See, for example, U.S. Pat. No. 6,400,003. The proposed structure in that patent, unfortunately suffers from several setbacks. First, the trench gate has a dual diffused channel on both sides of the trench gate. Second, the drain region extends entirely around the body portion.